1. Field of the Invention
The present invention relates to a method of forming an oxynitride film or the like and a system for carrying out the same. More specifically, the present invention relates to a method of forming an oxynitride film or the like on an object to be processed, such as a semiconductor wafer, and a system for carrying out the same.
2. Description of the Related Art
A semiconductor device fabricating process forms an insulating film on an object to be processed, such as a semiconductor wafer. This insulating film is used, for example, as a mask for impurity diffusion or ion implantation or as a source of an impurity. A silicon oxynitride film is used occasionally as such an insulating film. Silicon oxynitride films, as compared with prevalently used silicon oxide films, have a high dielectric constant and have a high capability of preventing penetration by an impurity, such as boron.
A silicon oxynitride film is formed on a surface of a semiconductor wafer by, for example, subjecting a semiconductor wafer to a thermal process. This thermal process will be described. A semiconductor wafer, such as a silicon wafer, is placed in a thermal processing device. The semiconductor wafer is heated to a high temperature of, for example, 900° C. Then, a process gas, such as dinitrogen oxide gas (N2O gas) is supplied into the thermal processing device for a predetermined time to form a silicon oxynitride film on the surface of the semiconductor wafer.
The progressive miniaturization of semiconductor devices requires reduction of thickness of silicon oxynitride films. Generally, it is preferable to lower the process temperature in the thermal processing device to form silicon oxynitride films of a small thickness, because the lowering of the process temperature is effective in reducing oxidation rate.
However, if the process temperature is lowered, for example, from 900° C. to 800° C. or 750° C., nitrogen gas cannot satisfactorily be pyrolyzed and, consequently, it is difficult to form an oxynitride film having a desired nitrogen content.
Methods of forming a silicon oxide film (SiO2 film) on each of a plurality of semiconductor wafers (hereinafter referred to simply as “wafers”) placed in a batch-processing furnace by oxidizing a silicon film on each wafer are classified into: dry oxidation methods that use oxygen gas (O2 gas) and hydrogen chloride gas (HCl gas); and wet oxidation methods that produce steam by burning oxygen gas and hydrogen gas (H2 gas) by an external device and that supply the steam and oxygen gas into a reaction tube. A suitable oxidation method is selected according to desired film quality.
The dry oxidation methods oxidize a silicon film with oxygen gas and remove impurities from the surface of the wafer by means of gettering-effect of chloride. More concretely, a wafer boat holding a plurality of wafers in a tier-like manner is carried into a vertical reaction tube, a process atmosphere in the reaction tube is heated by a heater surrounding the reaction tube, a process gas of an ordinary temperature including oxygen gas and hydrogen chloride gas is supplied through a ceiling part of the reaction tube into the reaction tube, and the process atmosphere is exhausted through a lower part of the reaction tube.
Higher process temperatures are more apt to produce a defect called a slip. In addition, it is preferable to avoid thermally affecting underlying films and to reduce energy consumption. Therefore, various studies have been made to reduce process temperature.
Since a diameter of the wafer is increasing progressively, thickness uniformity of a film formed on the surface of the wafer, i.e., intrasurface thickness uniformity becomes worse when the process temperature is reduced. In addition, thickness difference between films formed on the surfaces of the wafers, i.e., interwafer thickness uniformity also becomes worse.
It has been found, through examinations of relation between the position of a wafer on a wafer boat and the thickness of a film formed on the same wafer, that the thickness uniformity of films formed on wafers held in an upper part of the wafer boat is worse than that of films formed on wafers held in a lower part of the wafer boat. The inventors of the present invention infer that dependence of thickness uniformity on the position of the wafer on the wafer boat is due to the following reasons. FIGS. 19A to 19C show typically a flow of a gas over a wafer W, a temperature of the wafer W and a thickness of a film formed on the wafer W, respectively. Oxygen gas and hydrogen chloride gas flow from a periphery (edge) of the wafer W toward a center of the same. Then, oxygen gas oxidizes silicon on the wafer W as the same flows along the surface of the wafer W. Since the wafer W dissipates heat through a peripheral part thereof, the temperature of the wafer W increases toward the center of the wafer W. High temperature promotes the oxidation, and hence silicon on a central part of the wafer W is oxidized at an oxidation rate higher than that at which silicon on a peripheral part of the wafer W is oxidized. Consequently, even if the film is formed in a highly uniform thickness, there is a tendency for a part of the film on a central part of the wafer W to be thicker than a part of the same on a peripheral part of the wafer W.
Although it is only a small amount, interaction between hydrogen, which has been produced through decomposition of hydrogen chloride, and oxygen produces steam. The gas around an upper part of the wafer boat is not heated sufficiently. Thus, the temperature of the gas rises as the same flows from the periphery toward the center of the wafer W. Consequently, the amount of steam produced around the center of the wafer W is greater than that of steam produced around the periphery of the wafer W. The steam is effective in increasing the oxide film. Thus, the difference between the amount of steam produced around the peripheral part of the wafer W and that of steam produced around the central part of the wafer W greatly affects the difference between the thickness of a part of the film formed on the peripheral part of the wafer W and that of a part of the film formed on the central part of the wafer W. Consequently, the thickness of the part of the film on the central part of the wafer W is further increased so that the thickness of the film formed on the wafer W has a distribution of an upward convex curve, that is, the uniformity of the film thickness becomes worse. Since the temperature of the gas increases as the gas flows toward the lower part of the reaction tube, the above steam generating reaction is substantially equilibrated around the lower part of the wafer boat. That is, the gas is decomposed completely and all the possible amount of steam is produced before the gas flows along the wafers W. Therefore, substantially the same amount of steam exists around the peripheral part of the wafer W and around the central part of the wafer W as the process gas flows from the periphery toward the center of the wafer W and, consequently, the film is formed in a highly uniform thickness. Thus, it is inferred that the uniformity of the thickness of the films formed on the wafers held in the upper part of the wafer boat is considerably bad, and the difference between the thickness of the films formed on the wafers held in the upper part of the wafer boat and that of the films formed on the wafers held in the lower part of the wafer boat is great. Accordingly, it is difficult to lower the process temperature at the present.
A semiconductor device fabricating apparatus forms a thin silicon nitride film on an object to be processed, such as a semiconductor wafer. The silicon nitride film is excellent in insulating performance and corrosion resistance, and is used prevalently as an insulating film, as a means for impurity diffusion and as a mask for ion implantation. The silicon nitride film is formed on a semiconductor wafer by, for example, a CVD process (chemical vapor deposition process).
When forming a silicon nitride film on a semiconductor wafer, such as a silicon wafer, by the CVD process, the semiconductor wafer is placed in a thermal processing apparatus. Subsequently, an interior of the thermal processing apparatus is evacuated to a predetermined pressure of, for example, 133 Pa (1 Torr), and is heated to a predetermined temperature in a range of, for example, 650 to 700° C. Then, process gases, such as dichlorosilane gas (SiH2Cl2 gas) and ammonia gas (NH3 gas), are supplied into the thermal processing apparatus for a predetermined time in order to deposit a silicon nitride film on a surface of the semiconductor wafer.
The silicon nitride film thus formed has a refractive index RI=2.0 and has a substantially stoichiometric composition.
When forming the silicon nitride film, it is desired to use a low process temperature. However, ammonia cannot be satisfactorily decomposed and the silicon nitride film cannot be satisfactorily deposited if the process temperature is as low as 600° C., because ammonia has a high decomposition temperature. The inventors made various studies to use trimethylamine (TMA) having a decomposition temperature lower than that of ammonia, instead of ammonia, as a source of nitrogen.
A silicon nitride film formed on a semiconductor wafer by using a process temperature of for example 550° C. and trimethylamine as a source of nitrogen had an RI=2.9, which proved that the silicon nitride film was not satisfactorily nitrided. Such unsatisfactory nitriding is due to a large heat capacity of trimethylamine and hence difficulty in heating trimethylamine. Trimethylamine has a constant-pressure heat capacity (constant-pressure molar heat capacity) at 550° C. of 190 J/mol·K, which is about four times the constant-pressure heat capacity of 50 J/mol·K of ammonia. Under the above nitriding condition, deposition rate was as low as 0.27 nm/min, which proved that the silicon nitride film forming process using trimethylamine is not suitable for mass production.
A semiconductor device fabricating apparatus forms a silicon dioxide film on an object to be processed, such as a semiconductor wafer, by means of a chemical vapor deposition process (CVD process) or the like.
When forming a silicon dioxide film on a semiconductor wafer, such as a silicon wafer, by the CVD process, the semiconductor wafer is placed in a thermal processing device. Subsequently, an interior of the thermal processing device is evacuated to a predetermined pressure in a range of, for example, 13.3 Pa (0.1 Torr) to 1330 Pa (10 Torr), and is heated to a predetermined temperature in a range of, for example, 700 to 900° C. Then, process gases, such as dichlorosilane gas (SiH2Cl2 gas) and dinitrogen oxide gas (N2O gas), are supplied into the thermal processing device for a predetermined time. Thus, the dichlorosilane is oxidized, and a silicon dioxide film is deposited on a surface of the semiconductor wafer.
The silicon dioxide film thus formed is dense, excellent in insulating performance and resistant to peeling.
However, when forming the silicon dioxide film on the semiconductor wafer by the aforesaid chemical vapor deposition process, the silicon dioxide film is deposited on the semiconductor wafer at a low deposition rate.